This invention relates to an exchange device for a filter provided in a molten resin extrusion passage of an extrusion molding machine for removing impurities included in the molten resin.
In processing thermoplastic resins into sheets or films by an extrusion molding machine (extruder), a variety of filters are employed in the passage of the molten resin, such as between an extruder head and a metal mold or die, for preventing impurities or deteriorated resin in the extruder from being mixed into the products.
For exchanging these filters, there is so far known a device in which, for continuous operation of the extruder, at least two filters are provided in a casing and alternately employed for filtration, with the flow of the molten resin being switched by a valve so that alternate filters are exchanged, or a device in which the filter is built into a so-called cassette slide which is caused to slide in a direction normal to an extruder screw shaft to take out the used and contaminated filter and introduce a new fresh filter into the stream of the molten resin.
These known devices are disclosed in, for example, the Japanese Patent KOKAI Publication Nos. 60-174627 or 57-2741.
However, when the molten resin is apt to be deteriorated in quality on contact with air, it is quite likely that ambient air will intrude into the extruder with installment of a new filter at the time of the filter exchange by the filter exchange device, so that oxides of the resin may be formed within the extruder.
It results in that these oxides are mixed into the product after the filter exchange to form fish eyes. By the term "fish eyes" are meant flaws or defects on the appearance of the molded product demarcated from that of the acceptable product and which is caused by the presence of impurities, unmolten resin or deteriorated resin in the extruder having been mixed into the molded products during extrusion molding. For possibly preventing the occurrence of these "fish eyes", it is necessary that various components of the extruder, such as the metal mold, die or piping be dismantled and rinsed completely, or the inner components of the extruder be cleaned and flushed by continuing the extrusion of the resin until complete disappearance of the fish eyes. This means a significant loss not only in the molten resin but in the operating time until re-initiation of the regular molding operation.